U.S. Pat. No. 4,269,875 which issued in 1981 describes a novel method for preserving timber pioneered by Carl C. Beckgaard of Denmark. This method involves the insertion of solidified water soluble wood preservative rods into holes drilled in timber. The rods are made from boric oxide. The solidified rods have numerous advantages. They are easier to use in solidified form, gloves are not required. The wood preservative, being water soluble, is released over time by moisture in the wood. The strength of the wood preservative on a weight basis is increased, as the moisture in the wood combines with the boric oxide and produces boric acid in a ratio of 1:1.45.
The formation of boric oxide into water soluble rods is a three step process. The first step involves melting the boric oxide. The second step involves moulding the boric oxide into the desired shape. The third step involves annealing the rods to make them suitable for the intended use.
In U.S. Pat. No. 4,269,875 Carl C. Bechgaard observes that some fungi are controlled more effectively by copper oxide or one of the other metal oxides, in combination with boric oxide. He contemplated making the rods from a combination of boric oxide and copper oxide. In the fifteen years since the issue of the patent neither Mr. Bechgaard, nor any of his licensees have been successful in fabricating a rod that contains a combination of boric oxide and copper oxide.
The addition of copper oxide changes the physical properties of boric oxide, creating numerous problems. Some of the problems are relatively minor, such as the mixture sticking to the mould. Other problems are of critical importance, such as bubbling, crystallization or a failure of the mixture to anneal. The result has been failed attempts that are totally unsuitable for use. These failed rods tend to crumble into dust.
Boric oxide and copper have been combined in other forms. For example, wood preservative pastes exist that contain both boric oxide and copper. These wood preservatives utilize solvents. This methodology is not transferable to the manufacture of wood preservative rods. The use of solvents is precluded due to the low flash point of such solvents.